Transmission system



y 5, 1947. R. H. HERRICK Re. 22397" TRANSMISSION SYSTEM 7 2 Sheets-Shee1 Original Filed Aug. 14, 1939 mmi mQ v9 INVENTOR. Roswell HHer/"l'ckJuly 15, 1947.

R. H. HERRICK TRANSMISSION SYSTEM 2 Sheets-Sheet 2 0 m m m w m MH H wa AN8 w MN .8 Md m. m, wfiq wmwm NEUKPU NJ m qwucfiam @MN Y E8 35mm .WQQN wmQN Reissued July 15, 1947 TRANSMISSION SYSTEM Roswell H. Herrick, OakPark, Ill., assignor to Automatic Electric Laboratories, Inc., acorporation of Delaware Original No. 2,371,291, dated March 13, 1945,Serial No. 404,136, July 26, 1941, which is a division of Serial No.290,090, August 14, 1939. Application for reissue July 26, 1945, SerialNo.

11 Claims. 1

The present invention relates generally to im provements in signalcurrent transmission systems of the type in which signal controlledswitching circuits are provided for partially or completely blocking,under certain conditions, certain of the signal current channelsincluded therein and, more particularly, to improvements in telephonesubstation circuits havin incorporated therein coupled signal currentchannels for the transmission of incoming and outgoing signal currents.This application is a division of copending application Serial No.290,090, filed August 14, 1939, Roswell H. Herrick, now Patent No.2,282,405, granted May 12, 1942.

In the usual telephone substation circuit a hybrid system or antisidetone impedance network is provided for preventing signal currentsdeveloped during operation of the transmitter from being transmitted tothe receiver and for similarly preventing signal currents incoming overthe line extending to the substation from being transmitted to thetransmitter for reproduction. In this type of arrangement one of thefactors which determines the efiiciency of the side tone suppression isthe impedance of the talking circuit established by way of two connectedsubscribers lines. This impedance is not the same for any two differentestablished connections, and, accordingly, the hybrid system of eachsubstation is usually balanced to provide maximum side tone suppressionefliciency for average line conditions. In installations whereinamplification of incoming and outgoing signal currents is required, as,for example, in executive loud speaking sets, the conditions ofunbalance introduced in the substation circuit by the impedances of thelines over which an established connection extends may becomeintolerable. This is particularly true in substation installationsprovided in an exchange area Where the subscribers lines are of widelydifferent lengths. In order completely to obviate or to minimize thesinging which may result due to unbalance of the substation circuitoccasioned by unfavorable line conditions, signal controlled switchingmeans may be provided in the substation circuit for selectively blockingthe channels when not in use. In the usual arrangement of this characterthe signal channels are completely blocked when not in use. Moreparticularly, during intervals when the transmitting means of thesubstation is being used to transmit outgoing signal currents, theincoming signal current channel is rendered completely inactive.Conversely, during those periods when signal currents are incoming tothe substation, the transmitting or outgoing signal current channel isrendered completely inactive. An arrangement of this character, whilesatisfactory in operation, prevents any interruption of a speech trainbeing transmitted in one direction in response to signal currentstransmitted in the opposite direction. In other words, conversationbreak-ins are positively prevented.

It is an object of the present invention to provide improved telephonesubstation apparatus of the character described wherein the channels arepreferably only partially blocked when not in use, the blocking of thesignal current transmitting and receiving channels is accomplished in asimple and reliable manner, and wherein the control circuits arearranged to be exceedingl fast in operation so that speech clipping isminimized.

It is another object of the invention to provide an improvedtransmission system particularly suited for use in telephone substationcircuits of the character described wherein the control or channelblocking circuit is arranged in an improved manner to utilize gaseousdischarge tubes of the three-electrode type, whereby the circuit isrendered exceedingly fast in operation to effect channel blocking andunblocking operations Without any substantial speech clipping.

It is still another object of the invention to provide an improved andexceedingly simple arrangement for deionizing the gaseous dischargetubes each time the transmission of signal currents over either of thetwo channels is arrested.

It is a further object of the invention to provide an improvedtransmission system of the character described wherein the screenelectrode of one of the amplifier tubes in each channel is utilized asthe control element for blocking and unblocking the associated channel.

The novel features believedto be characteristic of the invention are setforth with particularity in the appended claims. The invention, both astoitsorganization and method of operation, together with further objectsand advantages thereof, will best be understood by reference to thespecification taken in connection with the accompanying drawings inwhich Figure 1 illustrates a substation circuit having incorporatedtherein certain of the features of the invention as briefly outlinedabove; Fig. 1a illustrates a slightly different arrangement of certainof the circuit elements shown in Fig. 1; and Fig. 2 illustrates amodified form of the substation circuit shown in Fig. 1.

Referring now more particularly to Fig. 1 of the drawings, thesubstation circuit there illustrated is adapted to be connected to atelephone line terminating at the terminals I which may, for example,form a part of a conventional automatic or manual telephone system, inwhich case, the distant end thereof will terminate in a line circuit thecharacter of which is determined by the character of the exchange.Briefly described, the circuit comprises a transmitter or microphone HMand receiving means in the form of a loud speaker I02 which are adaptedrespectively tobe coupled t an antiside tone impedance network includinga hybrid coil I03 and a balancing circuit I04 by means of signaltransmission channels I05 and I06. The outgoing signal current channelI05 comprises a vacuum tube amplifier I08 including three stages ofamplification respectively including the amplifier tubes I09, H0 andIII, the first of which is adapted to receive signal currents from themicrophone I0.I and the last of which is arranged to deliver its outputto the hybrid system I03. More particularly, the microphone IOI iscoupled to the input electrodes of the first amplifier tube I09 througha coupling network which includesa condenser II2 and an adjustablevoltage dividing or signal level control resistor I I3. The outputelectrodes of the tube I09 are coupled to the input electrodes of thesucceeding amplifier tube I I0 through a resistance'capacitance couplingnetwork which includes a pair of resistors H4 and H5 and a couplingcondenser IIB. Similarly the output electrodes of the second amplifiertube 0 are coupled to the input electrodes of the final amplifier tube III through a resistance capacitance coupling network which includes aresistor I", ,a voltage dividing or signal level adjusting resistor II 8and a coupling condenser H9. In order to bias the control electrode ofthe tube I09 to the proper negative potential with respect to theassociated cathode, thereis provided in the cathode leg of the outputcircuit of this tube a pair of series connected cathode biasingresistors I20 and I2I which are shunted by a signal current by-passcondenser I22. The control grid of the tube H0 is similarly biased tothe proper negative potential with respect to its associated cathode bymeans of a cathode biasingcircuit comprisinga resistor I23 shunted by asignal current by-pass condenser I24. A similar cathode biasing networkcomprising a resistor I25 shunted by a by-pass condenser .I26 isincluded in the cathode leg of the output circuit of the final amplifiertube III. Anode potentials are supplied to the anodes of the tubes I09,H0 and III from a source ofanode current, not shown, but having itspositive terminal connected to the terminal marked +B, over circuitswhich respectively include the resistors H4, H1 and certain of thewindings of the hybrid coil I03. Screen potentials are impressed uponthe screen electrodes of the indicated tubes from the anode currentsource over paths respectivel including-filter networks, that leading tothe screen electrode of the tube I09 including a resistor I21 and asignal current by-pass condenser I28 and that extending to the screenelectrode including a, resistor I29 anda bypass condenser I30.

The incoming signal current transmission channel I06 has includedtherein an amplifier I35 which includes two stages of amplificationrespectively comprising the amplifier tubes I35 and I31. Morespecifically, the incoming signal current terminals of the hybrid coilI03 are coupled to the input electrodes of the first amplifier tube I136through a coupling'network which includes a transformer I38 and anadjustable Voltage i ding or signal level control resistor I39. Theoutput electrodes of this tube are coupled to the input electrodes ofthe second amplifier tube I31 through ,a resistance capacitance couplingnetwork which comprises a resistor- 140, a coupling condenser MI and anadjustable voltage dividing or signal level control resistor I43. Theoutput electrodes of the second amplifier tube I31 are, in turn, coupledto the voice or signal current coil of theloud speaker I02 through acoupling transformer I44 and a T connected signal current level controlresistance network comprising a pair of series resistors I45 and I45 anda shunt resistor I46. In order to bias the control electrode of the tubeI31 to the proper negative potential with respect to the associatedcathode, there is provided in the cathode leg of the output circuit ofthis tube a biasing network which includes a resistor I41 shunted by asignal current by-pass condenser I48. Control of the bias impressed uponthe control electrode of the first amplifier tube I35. is effectedthrough the provision of a cathode biasing network which includes a pairof resistors I40 and I50 which are shunted by a signal current 'by-passcondenser I5I. Anode potentials are impressed upon the anodeelectrodes'of the two tubes I36 and I3'I from the source of anodecurrent over circuits respectively including a resistor I40 and theprimary winding of the coupling transformer I44. Screen potential isimpressed upon the screen electrode of the tube I36 from the source ofanode current over a path which includes afilter network comprising aresistor I52 and a bypass condenser I53. Screen potential for the screenanode of the tube 31 is derived in a manner more fully explainedsubsequently.

In order to control the gains of the amplifiers I08 and I35, thereby tovary the signal current transmission efficiencies of the channels I05and I-06 in accordance with'signal current fiow therethrough there isprovided a control circuit I55 which includes two control units, thefirst of which operates in response to signal currents traversing thechannel I05 and the second of which operates in response to signalcurrents traversing the channel I06. More particularly, the first unitof the control circuit comprises a gaseous discharge tube I56 of thethree-electrode type having an anode-I51 anda cathode I58 separated todefine a space current path and a control or start electrode I59interposed therebetween to control the flow of current over the spacecurrent path. The'startelectrode I59 is normally biased negatively withrespect to the cathode I58 over a path which serially includes aresistor IEO, a source of bias voltage IBI, a resistor I62 and aresistor I63. The space current path of the discharge tube I56 isincluded in the output circuit of this tube in series with the source ofanode potential mentioned above and the resistors I50 and I63, wherebycontrol voltages are developed across these resistors when adischarge'through the tube is established, which voltages are utilizedto control the final amplified tube III included in the channel I05 andthe first amplifier tube I36 included in-the channel I06. Moreparticularly, the voltage developed across the two series connectedresistors I50 and I-53in response to space current traversing the tubeI56 is utilized as the screen potential for the amplifier tube I I I,whereby this amplifier tube is only rendered operative to transmitsignal currents therethrough in response to a flow of current over thespace current path of the tube I50. Since the resistor I50 is alsoincluded in the cathode biasing circuit of the amplifier tube I36, thevoltage across this resistor also functions to control the signalcurrent gain of the amplifier I35. The two serially connected resistorsI50 and I63 are heavily by-passsed for signal currents through theprovision of a low impedance condenser I64. In order to impress a startpotential upon the start grid I59 of the discharge tube I56 when signalcurrents traverse the channel I05 a rectifying unit is provided whichincludes a three-electrode vacuum tube I65 having it input electrodescoupled to the channel I05 at a point following the amplifier tube H andover a circuit which includes a coupling condenser I66. The tube I65 isnormally biased to operate as a half wave rectifier by means of a biasbattery I61 connected between the input electrodes thereof. The outputelectrodes of the rectifier tube I65 are included in an output circuitwhich comprises the two resistors I50 and I62 and the source of anodecurrent connected to the terminal marked +B. In order momentarily tointerrupt the output circuit of the gaseous discharge tube I56 whensignal current flow over the channel I is arrested, there is provided aslow-to-release relay I68 having its winding included in the outputcircuit of the tube I 65 to be energized by space current traversingthis tube. This relay is provided with an armature I69 which is arrangedto interrupt the output circuit of the discharge tube The second controlunit embodied in the control circuit I55, namely, that associated withand responsive to signal currents traversing the channel I06 isessentially identical to that just described and provided in associationwith the channel I05. More particularly, the second control unitcomprises a, three-electrode gaseous discharge tube I10 having an anodeHI and a cathode I12 separated to define a space current path and astart electrode I13 interposed in this path for the purpose ofcontrolling the initial fiow of current thereover. The output electrodesHI and I12 of this tube are included in an output circuit which alsoincludes the resistor I2I, a second resistor I 14 and the source ofanode current connected to the terminal marked +B. The voltage developedacross these two series connected resistors when space current flowsthrough the tube I10 is utilized as the operatin potential for thescreen electrode of the amplifier tube I31 included in the channel I06,these two resistors being heavily bypassed for signal currents by a lowimpedance condenser I15 connected in shunt therewith. Since the resistorI2I forms a part of the cathode biasing circuit for the first amplifiertube I 09 included in the outgoing signal current channel I05, thevoltage across the resistor serves to determine the gain of theindicated amplifier tube and thus the signal current tranmissioneificiency of the channel I05. The start electrode I13 of the dischargetube I10 is normally biased negatively with respect to its associatedcathode over a path which includes a resistor I16, a source of biasvoltage I11, a, second resistor I18 shunted by a signal current by-passcondenser I19 and the resistor I14. For the purpose of impressing astart potential upon the start electrode I13 when signal currentstraverse the channel I06, a rectiher is provided which comprises athree-electrode vacuum tube I80 having its input electrodes coupled tothe channel I06 at a point following the amplifier tube I36 over acircuit including a coupling condenser I8I. This tube is normally biasedto operate as a half wave rectifier by means of a bias battery I82connected between the input electrodes thereof. The output electrodes ofthe tube I are included in an output circuit which serially comprisesthe resistors I2I and I18. In order to control the deionization of thedischarge tube I10 there is provided a sloW-to-release relay I83 havingits operatin winding included in the output circuit of the tube I80 andprovided with: an armature I84 which is operative to make and break theoutput circuit of the discharge tube I10".

Referring now more particularly to the operation of the systemillustrated in Fig. 1, it is pointed out that in order to condition thesignal current channels for signal current transmission, the cathodeheater circuit, not shown, for the various tubes i1lustrated,.is firstclosed. Witlr the apparatus in this condition the amplifier stagesrespectively comprising the tubes I09, H0 and I36 are all biased tooperate at normal gain.. Since, however, the two discharge tubes I56 andI10 are at this time in a deionized condition, no screen potentials areavailable for the screen electrodes of the final amplifier tubes III andI31 respectively included in the amplifiers I08 and I35. Thus, bothchannels are effectively blocked so that no circulation of energy aroundthet system can occur. If, with the apparatus in this condition, a userof the substation circuit speaks into the microphone IOI, the resultingsignal voltage is impressed upon the input electrodes of the amplifiertube I09 and the amplified output voltage is, in turn, impressed uponthe input electrodes of the tube I I0. As further amplified by the tubeIIO the signal voltage is impressed across the input electrodes of theamplifier stage II I and also through the condenser I66 across the inputelectrodes of the rectifier tube I65. During alternate half cycles ofthis voltage the bias potential impressed upon the control electrode ofthe tube I65 by the battery I61 is overcome so that space current flowsthrough this tube over a circuit which includes the winding of the relayI68, the resistor I62 and the resistor I50. As indicated above, duringsignal current transmission through the channel I05, due to themagnitude of the bias impressed across the control electrodes thereof bythe battery I61, the vacuum tube I65 functions as a half wave rectifier.Accordingly, a pulsating direct current traverses the space current pathof this tube and the resistors I62 and I50. Due to the smoothing actionof the condenser I62 shunting the resistor I62, however, the voltageacross this resistor does not fall to a zero value during alternate halfcycles of signal current but is maintained at a value sufiicient topermit ionization of the discharge tube I56. The resulting voltagedeveloped across the resistor I62 is impressed across the inputelectrodes I58 and I59 of the discharge tube I56 in opposition to thenegative bias voltage afforded by the Cbattery I6I. This voltage is ofsuflicient magnitude to lower the potential of the start grid I59 to apoint where a discharge through this tube occurs. When the tube I56 isionized in this manner, current is caused to traverse the space currentpath thereof; this current also flowing through the series connectedresistors I50 and I63. The last-mentioned resistor has a resistancevalue substantially greater than that of the resistor I50 and alsosubstantially greater than the resistance of the space current paththrough the tube I56. Accordingly, a substantial portion of theavailable voltage of the anode current supply source appears as avoltage drop across the re- 7 sister I 63. This voltage drop, asaugmented by the vbltage drop across the resistor I50, is impressed asan operating potential upon the screen electrode of the final amplifiertube III included in the channel I05, thereby to render the tube HIoperative to transmit the generated signal currents therethrough andthrough the hybrid coill'03 to the line terminating at the terminals I60. -As indicated above, the operations just described all occur duringthe first cycle of the signal current developed in response to the soundwaves impinging upon the diaphragm of the microphone -IOI. Accordingly,no substantial clipping of the first syllable of the speech representedby the sound waves occurs. Simultaneously with the activation of theamplifier tube II I, the first amplifier tube I36 of the channel I66 iseither biased to have a very low gain therethrough or beyond cut-off,depending upon the constants of the control circuit. Thus, the currenttraversing the resistor I 50 and flowing through the space current pathsof the tubes I56 and [65 causes a voltage to be developed across thisresistor which is impressed through the resistor I49 upon the cathode ofthe tube I36 as a positive bias. This bias voltage has the effect oflowering the gain ofthe tube I36 and if of sufficiently large magnitudemay serve to bias this tube to a point beyond anode current cut-oiT suchthat the channel I06 is completel blocked in the first amplifier stagethereof. Alternatively, if the resistance value of the resistor I50 ischosen so that the voltage developed thereacross by the space currentsflowing therethrough does not exceed the anode current cut-01f value ofthe tube I36, this tube w-illpass incoming signal currents to thesucceeding amplifier tube I31 at a substantially reduced gain. By virtueof the arrangement just described if the antiside tone networkcomprising the hybrid coil I03 and the balancing circuit -I'0'4 aregreatly unbalanced due to the condition of the lines connected to theterminals I00, such that signal currents are transmitted from thechannel I05 through this network to the input side of the channel I06,the partial or complete blocking of the latter channel in the firstamplifier stage comprising the tube I36 prevents the signal currentsfrom being fed through the channel to initiate operation of the secondcontrol unit comprising the rectifier tube 186 and the-discharge tubeI10.

:From the foregoing explanation it will be apparentthat the currenttraversing the space current path of the tube I65 as a result of signalvoltage impressed upon the input electrodes of this tube also traversesthe operating winding of 'theslow-to-release relay I68. Shortl followingthe first few cycles of this voltage, the relay I68 operates to break,at its armature I69 and the associated back contact, the previouslytraced output circuit including the space current path of the dischargetube 56. This circuit is recompleted at the armature I69 and itsassociated workin contact when the armature I69 is moved to itsattracted position, During the interval required for operation of thearmature I69 from it retracted position to its attracted position theoutput circuit of the tube I56 is held open. Accordingly, the "flow ofspace current through this tube and through the resistors I50 and I63 ismomentarily interrupted, causing the final amplifier tube III of thechannel I05 momenitarily to be rendered inoperative and the bias voltageimpressed across the input electrodes of thetfirst amplifier tube I36included in the channel I66 momentarily to'be returned to the normalvalue. During this operation the potential of the start electrode I59 ismaintained at a value permitting the flow of space current through thetube I56 due to the continued operation of the rectifier tube I 65.Accordingly, immediately the output circuit of the discharge tube I56 isrecompleted at the armature I69 and its associated front contact, spacecurrent again fiows through the tube I56, thereby to render the finalamplifier tube III operative and partially or completely to block thesignal transmission channel I06. During the interval when the armatureI69 is operating between its two positions no signal currenttransmission through the channel I05 can occur. Accordingly, a veryslight clipping of the intermediate syllable of the speech train mayoccur during this switching interval. Such clipping is ofno consequence,however, since the listener, upon hearing the first syllables of thespeech train and the syllables following the interval during whichclipping may occur, can readily ascertain, by interpolation, thesyllable or partial syllable Which is cut off during the switchingoperation.

When the flow of signal currents through the channel I05 is arrested,the flow of current through the space current path of the rectifier tubeI65 is reduced to zero or to a negligible value, Accordingly, thepotential impressed upon the start electrode I59 of the discharge tubeI56 is increased in a negative sense to a value which will preventfurther current flow through this tube after the tube is deionized, Asexplained above, the character of the discharge tube I 56 is such thatwhen ionization therein occurs th start electrode I59 has no furthercontrol over the current flow therethrough. Since, however, theoperating winding of the relay I68 is deenergized when the flow ofsignal currents through the channel I05 is arrested, this relay iscaused to restore after an interval. During the interval when thearmature I69 of the relay I66 is moving between its attracted andretracted positions theoutput circuit of the discharge tube I 56 isinterrupted in an obvious manner so that no voltage is availablefor'sustaining the discharge through the tube. Accordingly, the tube I56is deionized, at which time the control of the space current path in thetube is restored to the negatively biased start electrode I59. Due tothe negative potential impressed upon this electrode by the battery I6Iionization of the tube is prevented when the armature I69 is moved intoengagement with its associated back contact to again impress the voltageof the anode current source across the anode I51 and the cathode I56.When the space current flow through the two tubes I65 and I56 isinterrupted in the manner just explained, the voltage drops across theresistors I50 and I63 are reduce "substantially to zero. Accordingly,the operating potential is removed from the screen electrode of thefinal amplifier tube II I and this tube is rendered inoperative totransmit signal currents to the hybrid coil I03. Also, the value of thenegative bias impressed across the input electrodes of the amplifiertube I36 is decreased to normal so that this amplifier stage isconditioned to operate with normal gain.

The manner in which the second control unit comprising the rectifiertube F and the-discharge tube II'0 functions partially or completely toblock the transmission channel I05 and to activate the final amplifierstage of the channel I06 9 when signal currents are transmitted from theline terminating at the terminals I through the hybrid coil I03 to theinput side of the channel I06, is substantially identical to the mode ofoperation of the first control unit as described above. Moreparticularly, signal currents transmitted to the input electrodes of thetube I36 through the coupling transformer I38 and the voltage dividerI39 are amplified by this tube and the amplified signal voltage isimpressed across the input electrodes of the two tubes I31 and I80. Theresulting space current flow through the rectifier tube I80 causes theionization of the discharge tube I10, whereby a relatively heavy currentis caused to traverse the control resistors I2I and I14. The voltagedrop across these two serially connected resistors is utilized as anoperating voltage for the screen electrode of the tube I31, whereby thistube is rendered operative to amplify the signal currents and transmitthe same through the transformer I44 and the T-connected volume controlresistor network to the signal current coil of the loud speaker I02 forreproduction. The voltage developed across the resistor I2I is impressedas a negative bias between the input electrodes of the first amplifiertube I09 included in the channel I05, whereby this tube is eitherrendered completely inoperative to transmit signal currents therethroughor the gain of the tube is reduced to a very low value. Shortlyfollowing the initial flow of signal currents through the channel I06,the slow-to-release relay I83, which is energized by the space currentof the tube I80, operates momentarily to extinguish the flow of spacecurrent through the discharge tube I10 with the result that the channelI06 is momentarily blocked and the amplifier I09 is momentarilyreconditioned to operate with normal gain in the manner described abovewith reference to signal transmission through the channel I05. Duringthe transmission of signal currents through the channel I06 to the loudspeaker I02, the T-connected resistance network included in this channelbetween the coupling transformer I44 and the signal current coil of theloud speaker operates in a well-known manner to maintain the soundoutput level of the loud speaker substantially constant regardless ofchanges in the level of the signal currents transmitted to the inputside of the channel. When signal current transmission through thechannel I06 is arrested the flow of space current through the rectifiertube I80 is arrested. Thus, the operating winding of the relay I83 isdeenergized and the potential impressed upon the start electrode I13 ofthe discharge tube I10 is restored to a negative value which will notpermit ionization of the tube after the tube has been deionized. Afteran interval equal to the release period of the relay I83, the armatureI84 is'moved from its attracted position to its retracted position.During such movement, the output circuit of the tube I10 is interrupted,thereby to disconnect the source of anode voltage from the anode I1 I.As a result, the tube is deionized and control of the space current paththereof is restored to the negatively biased start electrode I13. Whenthe flow of space current through the two resistors I2I and I14 isarrested the potential impressed upon the screen electrode of theamplifier tube I31 is reduced substantially to zero, whereby this tubeis rendered inoperative. Also, the voltage drop across the resistor I2lis sufficiently reduced to permit the amplifier tube I09 of the channelI to operate with substantially normal gain.

The purpose of providing the relays I68 and I83, having slow-to-releasecharacteristics, for controlling the output circuits of the tubes I56and I10 respectively is to prevent clipping. More particularly, theperiod or interval required for the release of either of these tworelays prevents the blocking of the channels during momentary pauses insignal current transmission through the respective associated channels.To this end, the relay I68, for example, provides a hang-over period,during which the discharge through the tube I56 is sustained, whichexceeds the short pauses occurring in normal speech. Similarly, theslowto-release period of the relay I83 serve to sustain the flow ofspace current through the discharge tube I10 for intervals during whichshort speech pauses may occur.

A modified arrangement for obtaining the delay intervals just describedis illustrated in Fig. 1a, The relay arrangement illustrated in thisfigure may be used in lieu of of the relays I68 and I83 as shown in Fig.1 without changing the armature and contact spring assemblies. In theFig. 1a, arrangement the relay coil comprises a pair of difierentialyrelated windings, the lower winding of which is connected in series witha condenser l and a resistor I9I across the upper winding. Also, thelower winding is constructed to provide a substantially greater numberof ampere turns than the upper winding. With this arrangement includedin the circuit of Fig. l the initial rush of current through the upperwinding occasioned by a flow of space current through the tube I65, forexample, causes the relay armature rapidly to be attracted. During suchenergization, the condenser I90 charges relatively slowly so that theflow of current through the lower winding is relatively low and nosubstantial opposing action occurs. When the flow of current through theupper winding is decreased or completely arrested, the condenser I90discharges through the two windings of the relay in series so that theampere turns represented by the two windings are in aiding phaserelation and the armature of the relay is maintained in its attractedposition for a short time interval after the current flow through theassociated space current path ceases.

If the circuit constants of the circuit illustrated in Fig. 1 are soproportioned that each of the first amplifier stages respectivelycomprising the tubes I09 and I36 is not completely blocked when thecomplementary transmission channel is active, the arrangement permitswhat is known as conversation break-ins to occur. In this regard it willbe apparent that signal currents transmitted to the substation circuitover the line connected to the terminals I00 are passed through thehybrid coil I03 to the input side of the channel I06 at a reasonablyhigh intensity as compared with the currents transmitted from thechannel I05 to the channel I06 through the hybrid coil during operationof the microphone IOI. With the circuit so arranged that the gain of theamplifier stage comprising the tube I36 is only reduced to a low valueduring signal transmission through the channel I05, the incoming signalcurrents are transmitted at reduced gain through this first amplifierstage of the channel I06 and are impressed across the input electrodesof the rectifier tube I80, whereby the discharge tube I10 is ionized inthe manner previously explained. In response to the fiow of spacecurrent through the discharge tube I10 the required operating voltage isimpressed upon the screen nal voltages appearing across the outputcircuit I of this second tube are impressed across the input electrodesof the third amplifier tube 2 II and also across the input electrodes ofthe rectifier tube 265. The resulting space current fiow through thetube 265 causes a voltage to be developed across the resistor 281 whichreduces the negative potential upon the start electrod 259 of thedischarge tube 256 to a point where this tube becomes ionized andcurrent fiows through the two load resistors 250 and 263. The voltagedeveloped across the resistor 263 opposes the voltage of the 0 battery290 and is of such magnitude that the bias between the input electrodesof the tube 2| l is sufficiently reduced to permit this tube to operatewith normal gain. Accordingly, the amplified signal currents deliveredto the input electrodes of the amplifier tube 2 I l are amplifiedtherein and transmitted through the coupling transformer 285 and thehybrid coil 203 to the line extending to the terminals 200. The voltagedeveloped across th resistor 250, On the other hand, is impressed as anegative bias voltage between the input electrodes of the amplifier tube236,-thus reducing the gain of this tube. By suitably proportioning theresistance value of the resistor 250, the voltage developed thereacrosswhen space current traverses the discharge tube 256 may be proportionedto bias the tube 236 beyond cut-off such that the channel 206 iscompletely blocked, or to bias this tube so that it operates with only asmall gain, whereby only signal currents of substantial magnitudes maybe transmitted therethrough. By thus reducing the gain of the amplifiertube 236, signal currents which may be transmitted through the hybridsystem 203 to the input side of the channel 206, due to an unbalancedcondition of the hybrid system, are prevented from initiating theoperation of the control unit comprising the rectifier tube 280 and thedischarge tube 210. The relay 268 included in the output circuit of therectifier 265 functions, in the exact manner previously explained withreference to the corresponding relay of Fig. 1, momentarily to break thespace current path of the discharge tube 256 shortly following theinitial signal current fiow over the channel 205. During its operatingperiod, therefore, the relay 268 momentarily blocks the transmission ofsignal currents through the channel 205 and increases the gain of theamplifier 236 to its normal value. When, however, the relay 26B is fullyoperated, the ionized condition of the tube 256 is reestablished,whereby the channel 205 is again rendered operative and the gain of theamplifier tube 236 is again reduced to an exceedingly low value. Uponoperating, the relay 268 completes, at its armature 269', an obviouspath for shortcircuiting its upper winding, thereby to render itselfexceedingly slow to release in a manner well understood in the art. Byvirtue of this arrangement, delay or hang-over periods are provided toprevent the blocking of the channel 205 and the unblocking of thechannel 206 during the short intervals which occur between speechsyllables.

When the flow of signal currents through the channel 205 is arrested,space current flow through the rectifier tube 265 ceases. As a result.the relay 268 is deenergized and the potential upon the start electrode259 is restored to a value which will prevent space current fiow throughthe tube 256 after this tube is deionized. After an interval determinedby the release period of th relay 268, this relay restores to interrupt,at its armature 269', the path short-circuiting its upper winding,thereby to render itself reasonably fast to operate. At its armature269, therelay 268 momentarily interrupts the output circult of thedischarge tube 256, whereby this tube is deionized and control of thespace current fiow therethrough is restored to the start grid 255. Whenthe flow of space current through the tube 256 is arrested, the voltagedrops across the resistors 263 and 250 are reduced to values such thatthe amplifier tube 2| I is biased beyond anode current cut-off and theamplifier tube 236 is biased to operate with normal gain.

The mode of operation of the control unit comprising, the rectifier tube280 and the discharge tube 210 and associated with the channel 206 isidentical with that just described with reference to the unit associatedwith th channel 205. From the foregoing explanation it will be ap-parout that if the resistors 250 and 2'2l ar properly proportioned so thatthe associated amplifier tubes 236 and 2! are not biased beyond cut-offwhence space current traverses these resistors, each of the two channels205 and 206 is partially active when the other channel is in use.Accordingly, conversation break-ins may occur in a manner which will beapparent from the above description. In this regard it will be apparentthat in the usual installation complete blocking of each channel whenthe other channel is in use is not required in order to prevent singingdue to the substantial side tone suppression achieved through operationof the hybrid system 203.

While there has been described what is at present considered to be thepreferred embodiment of the invention, it will be understood thatvarious modifications may be made therein, and it is contemplated tocover in the appended claims all such modifications as fall within thetrue spirit and scope of the invention.

I claim: .1. In a transmission system, a signal current transmissionchannel including amplifying means, said amplifying means including astage comprising an electron discharge tube having a screen electrode, acontrol circuit having an input circuit coupled to said channel at apoint preceding said amplifier stage and comprising a gaseous dischargetube including an anode and a cathode separated to define a spacecurrent path and a start electrode for controlling the flow of currentover said path, means included in said input circuit for impressing aunidirectional start potential upon said start electrode when signalcurrents traverse said channel,

means controlled by the resulting flow of space current over said pathfor impressing an operating potential upon said screen electrode,thereby to render said amplifier stage operative, and relay meanscontrolled by said input circuit operative to lower momentarily thepotential difference between said cathode and anode of said gaseousdischarge tube when the flow of signal currents through said channel isarrested, thereby to arrest the fiow of current over said path and ingsaid source to normally maintain the said device in said receivingchannel in blocked condition and the said device in said transmittingchannel in operative condition, means responsive to the signal strengthimpressed on said receiving channel to efiect a condition, meansresponsive to the signal strength existent in the output line of saidtransmitting channel to eifect a condition in opposition to said firstmentioned condition, and means responsive to the balance between saidtwo cOnditions in control of the potential of said source whereby saidchannels are alternately conditioned for signal passage in accordancewith directional signal conditions.

9. Telephone substation apparatus comprising in combination a receivingamplifying channel and a transmitting amplifying channel each having aspace discharge device operative to alternately block and condition itsassociated chan nel, a control network for said devices, a signalstrength responsive device operative to effect a condition in proportionto the incoming signal strength impressed on said receiving channel, asecond signal strength responsive device operative to effect a conditionin proportion to the strength of output of said transmitting channel andin opposition to said first mentioned condition, means to control saidnetwork in accordance with the degree of balance between said twoconditions, and means to diminish the efiicacy of said first mentionedresponsive device upon increase of signal strength impressed on said second mentioned responsive device.

10. Telephone substation apparatus comprising in combination a receivingamplifying channel and a transmitting amplifying channel each having acontrol device therein operative to alternately block and condition thechannel, a control circuit for said devices Op r ive to effect theblocking of one channel while the other is conditioned and vice versa, asignal strength responsive device operative to eifect a condition inproportion to the incoming signal strength impressed on said receivingchannel, a second signal strength responsive device operative to effeeta condition in proportion to the strength of output of said transmittingchannel and in opposition to said first mentioned condition, means tocontrol said circuit in accordance with the degree of balance betweensaid two conditions, and means to diminish the efficacy of said firstmentioned responsive device upon increase of signal strength impressedon said second mentioned responsive device.

11. Telephone substation apparatus comprising in combination a pair ofconductors adapted to be connected to a line, a receiving amplifyingchannel coupled to said conductors, a transmitting amplifying channelcoupled to said conductors through a voltage transforming network, adevice in each of said channels operative to alternately block andcondition the channels, a control circuit for said devices operative toeffeet the blocking of one channel while the other channel isconditioned and vice versa, means to effect a condition in response tothe signal strength impressed on said receiving channel, means to effecta condition responsive to the signal strength existent in theinterconnection between said transmitting channel and said voltagetransforming network and in opposition to said first mentionedcondition, and means to control said circuit in accordance with thedegree of balance between said two conditions whereby said channels arealternately conditioned for signal passage in accordance withdirectional signal conditions.

ROSWELL H. HERRICK,

